A bacteriolysin efficient against isteria monocytogenes

Abstract:

Lactococcus carnosus CNCM I-4031 is a psychrotrophic lactic acid bacterium used for the biopreservation of seafood products. It effectively inhibits the growth of pathogenic bacteria responsible for food spoilage, such as Listeria monocytogenes, through an atypical mechanism based on direct cell-to-cell contact, without producing antimicrobial compounds such as bacteriocins. The aim of this study was to clarify the mechanism of inhibition of this bacterium against undesirable bacteria using label-free LC-MS/MS shotgun proteomics and analysis of the expression of genes involved in the expression of cell envelope proteins in L. carnosus when grown alone and in co-culture with L. monocytogenes. The study identified a specific cell wall protein, called LYSO, which has a toxic C-terminal domain and exhibits peptidoglycan hydrolysis activity against L. monocytogenes. Further analysis using knockout mutants provided additional evidence for the involvement of LYSO in the inhibitory activity. These results suggest the important role of this bacteriolysin in the contact-dependent mechanism of L. carnosus against L. monocytogenes.

Context and challenges:

In the food industry, combating Listeria monocytogenes is a major challenge, particularly in ready-to-eat (RTE) foods, as this bacterium can cause severe listeriosis, with a high mortality rate.  L. carnosus CNCM I-4031 is a lactic acid bacterium whose mechanism of inhibition against L. monocytogenes is not based on the production and release of bacteriocins into the environment, but rather on direct contact between cells.  Understanding this mechanism is essential for optimizing the use of lactic acid bacteria in biopreservation, ensuring their efficacy and safety, and exploring innovative applications

Results :

Researchers have identified LYSO, a surface protein produced by Lactococcus carnosus, which specifically lyses the cell wall of L. monocytogenes during direct cell-to-cell contact. LYSO expression increases in co-culture, and the purified protein exhibits targeted lysozyme activity.  A mutant lacking this protein shows greatly reduced inhibitory activity, confirming its central role, although other factors also appear to be involved.  This is the first demonstration of a bacteriolysin acting in a contact-dependent inhibition mechanism in a Gram-positive bacterium.

Perspectives:

With a view to this work, the aim will be to gain a better understanding of LYSO's mechanism of action by exploring how LYSO is delivered and how the producing bacterium protects itself from it.  L. carnosus could be used as a biopreservation agent targeting Listeria in ready-to-eat foods. This food application could be the subject of biotechnological innovation to optimize or combine LYSO with other effectors to enhance its antimicrobial efficacy.